Directional parasitic antennas are an important electromagnetic device with wide application in the modern world. The best-known example of such arrays is the Yagi-Uda antenna. These are parasitic directional rays made from dipole or dipole like elements which are separated from each other by at least an eighth of a wave or more and are not attached directly to each other. When the elements are placed much closer than an eighth of a wave they interact to a point where it is not possible to use them in a directional manner. In other words Yagi-Uda antennas have the advantage of being directional at the expense of a prescriptive design approach.
It will be appreciated that because the elements interact within a Yagi-Uda antenna when placed very close to one another, is not possible to fully take advantage of one of the main aspects of electromagnetic science. That aspect is called evanescent wave transmission. It is also called plasmonic transmission.
In plasmonic transmission electromagnetic waves propagate over very short distances; usually a practical limit being about 1/10 of a wavelength. Beyond that distance other near field mechanisms tend to dominate and the plasmonic or evanescent mode dies off rapidly (e.g., exponentially).
What is needed therefore is a directional antenna array that takes advantage of the evanescent wave transmission, or plasmonic transmission, to produce a better performing directional antenna that either has better directionality and or smaller size.